In the following, silicon shall mean crystalline, potentially doped, silicon if not stated otherwise. A person skilled in the art will understand that a crystalline silicon chip or wafer, when used as a basis for a solar cell, will be purposively doped to make the silicon p-type or n-type. Further, a person skilled in the art will also understand that dielectric layers mentioned in the following, might, depending on the means of deposition, include elements not shown in the stoichiometric formula. For instance, if deposited by means of chemical vapour deposition, the various dielectric layers might include hydrogen originating from one or more of the precursor gasses. The person skilled in the art will also understand that said dielectric layers may be amorphous or crystalline, depending of the deposition conditions and means.
The following references to other publications are indicated with reference numerals in square brackets, whereby relevant text of such references are included as parts of this disclosure as they contain technical information that person skilled in the art may find useful for understanding the background of the present invention.
The key parameter ratio for producing cost-effective solar cells is the cost per watt of output effect, e.g. dollar per watt. There are two ways of reducing the cost per watt; by increasing the efficiency of a solar cell and by reducing the cost of production.
Good surface passivation with a low surface recombination velocity is a prerequisite for obtaining high efficiency in silicon solar cell devices in which high minority carrier lifetime is of essence. Several dielectric materials are known which can be used, either alone or in combination, to passivate the surface of a silicon wafer or chip for obtaining reduced surface recombination. Examples of such layers are silicon nitride (SiNx), amorphous silicon (a-Si), aluminium oxide (Al2Ox) and thermally grown silicon oxide (SiO2). Further, stacked combinations of two or more of the mentioned dielectric layers, such as SiNx/a-Si; SiNx/SiO2 and SiNxAl2Ox, have also been shown to provide good surface passivation quality.
During the manufacturing of crystalline silicon-based solar cells, the solar cells are usually exposed to one or more process steps at high temperatures, typically in the range of 800° C. and above. One such process step is the firing, i.e. activation, of printed contacts to make a good connection between the contacts and a p-n junction provided in the wafer. In the presence of any dielectric passivation and/or anti-reflection layers, the contacts will typically have to be fired through the mentioned layers, entailing that the dielectric layers have to withstand the high temperature without losing the qualities enabling its intended purpose.
A-Si, alone or in a stack with SiNx or SiO2, has been shown to give a close to perfect passivation of a crystalline silicon surface. However, studies have shown that a-Si loses its passivation properties if heated to above 500° C. [1]. A-Si also has a very high optical absorption in the lower wavelength range of visible light, and any a-Si layer on the front of a solar cell may therefor “steal” an amount of the incoming light. Thermal oxidation of silicon might also provide good surface passivation. However, the growth of such a SiO2 layer requires high temperature over a prolonged period of time, which is unwanted for low-cost production due the amount of energy required for heating. In addition, the thermal budget also increases diffusion of impurities in the silicon, which usually is of sub-electronic grade when used for solar cells. The impurity migration might significantly degrade the minority carrier lifetime in the silicon, and thus the efficiency of a silicon solar cell. SiNx has been shown to give a decent passivation of crystalline silicon, but when used on p-type silicon wafers there have been problems with parasitic shunting due to the high positive charge in the SiNx layer [2]. More recently, Al2Ox with negative charge has been shown to provide very good surface passivation for p-type crystalline silicon [3]. However, Al2Ox is usually deposited by means of atomic layer deposition (ALD), which requires very high vacuum, and which has been challenging to incorporate with the rate of mass production usually envisaged for solar cell manufacturing.
Silicon oxynitride (SiOxNy) has been shown to be a promising dielectric material for surface passivation of silicon [4, 5]. It has also been investigated to use SiOxNy in a stack with SiNx for surface passivation in photovoltaic applications [6]. However, the passivation quality reported so far has not been sufficient to obtain satisfactory low surface recombination velocities. Further, the thermal stability of SiOxNy has been a challenge, and the passivation quality has generally degraded after high temperature treatment, such as contact firing. Deposition temperatures of SiOxNy have generally been in range of 250° C. and above.